steep sides o Earthslides may block streams as a result of the 

 weathering of certain rock formations, and the saturation of soils 

 which allows the mass to slide from the steep slopes. Aerial photos 

 and ground reconnaissance indicate that approximately 250 years ago 

 a large slide blocked Harris River and changed its course. 



The choking of stream channels by material other than landslides is 

 of relatively canmon occurrence. High flood water undermines many 

 trees growing along the stream channel. Stream banks are cut in one 

 section and filled in another; new gravel bars are formed, others 

 are washed out. Evidence of this change can be readily seen in mud- 

 bars, cutbanks, in log tangles embedded in stagnant pools, and in 

 many other forms. Especially vulnerable to change are unstable streams 

 which are subjected to flash floods during periods of heavy precipita- 

 tion. This change is characteristic of many of the salmon streams 

 of Southeast Alaska, but the rate and extent of this change over 

 relatively short periods is not known. As it is not known to what 

 extent, if any, the process of change may be accelerated by logging, 

 the extent of change which occurs over relatively short periods of 

 time, both prior to and following logging, has been recorded on all 

 streams. 



Mapping of all test streams was begun in 1949, The following detail 

 was mapped in place on a scale of $0 feet to the inch, 



a. Size and number of logjams 



b. Number of windfalls and pieces of debris 



c. Character of streambed, i,e,, whether sand, gravel, 

 bedrock, etc, 



d. Streambank cutting 



e. Pools and riffles 



f . Gravel bars and stream channels 



g. Vegetative type along streambank 



Debris , The amount of debris in streams is cyclic in nature. Some 

 years a total accumulation takes place; other years extreme floods 

 may flush much of the debris out of the streams. The mapping project 

 indicated that the period 1949 through 1953 has been one of accumula- 

 tion. Heavy acc\jmulation of debris in critical areas is harmful in 

 that it may block the upstream migration of spawning salmon. Debris 

 may change the stream course and thus set up a reaction which is felt 

 for many miles downstream. 



The effect of debris and logjams in streams is not as yet fully known, 

 however. The presence of this material may be beneficial as well as 

 harmful. Stream barriers are generally transient in nature and new 

 channels are soon formed under or around these barriers. 



During the mapping process and spawning surveys it was observed that 

 salmon congregated in places where a protecting cover of debris 

 exLstedo Considerable variation takes place from year to year in 

 total amoimt of debris in these streams under natural conditions. 



35 



